Apparatus for spinning filaments of synthetic thermoplastic fiber-forming polymers



Jan. 3, 1967 H. SCHIPPERS 3,295,160

APPARATUS FOR SPINNING FILAMENTS OF SYNTHETIC THERMOPLASTIC FIBER-FORMING POLYMERS Filed July 21, 1964 FIGZ o CONSTANT a/a 0 Z :2, LINE T0 SPINNING com- 1755510 FEED PUMPS METER/N6 zozvs ZONE ZONE /2D j g 6 2 l5 4 Z +4, 220 D u m 270 D: sa/u'w DIAMETER L THREAD DEPTH 0on5 DIAMETER INVENTOR.

HEINZ SCHIPPERS x ATT'YS United States Patent Oflfice 3,295,160 Patented Jan. 3, 1967 APPARATUS FOR SPINNING FILAMENTS OF SYN- THETIC THERMOPLASTIC FIBER FORMING POLYMERS Heinz Schippers, Rernscheid-Lennep, Germany, assignor to Barmer Maschinenfabrik Aktienges, Wuppertal- Oberbarmen, Germany Filed July 21, 1964, Ser. No. 384,071

Claims priority, application Germany, Aug. 3, 1963,

6 Claims. Eon. 18-8 This invention relates to apparatus capable of spinning filaments of synthetic thermoplastic fiber-forming polymer, sometimes referred to as a spinning extruder, wherein the thermoplastic polymer is first melted and conveyed by an extrusion screw to a spinning pump or series of spinning pumps where the molten polymer is dosed into a spinneret for extrusion into filaments through a plurality of small orifices or openings corresponding to the desired diameter or denier of the filaments. More particularly, the invention is concerned with a specific improvement in the construction and dimensional relationships of the extrusion screw as one element of the spinning extruder.

In the production of synthetic thermoplastic polymer filaments or fibers, it is well known that an extrusion screw can be combined with one or more spinnerets so as to first melt the solid polymer, usually in the form of granules, particles or flakes, and then convey the molten polymer under pressure built up in the extrusion screw to the spinneret. The polymer can be melted in the extrusion screw itself or it can be premelted and then introduced into the screw for building up the desired extrusion pressure and producing a homogeneous melt. As the molten polymer is extruded from the orifices of the spinneret in filamentary form, the individual filaments are rapidly cooled and simultaneously stretched for molecule orientation. This combination of apparatus is referred to as a spinning extruder and generally includes a spinning pump of conventional construction, usually a gear wheel pump, associated with each spinneret. This spinning pump is located between the outlet of the extrusion screw and directly before the spinneret, with reference to the direction of flow of the molten polymer, and is designed to provide an accurate dosage of molten polymer into the spinning head of the spinneret. In essence, the desired function of the spinning pump is to maintain a constant pressure on the molten polymer a it is extruded from the spinneret and to make this pressure sufliciently independent of fluctuations in pressure caused by the melting and conveying operations of the extrusion screw. Otherwise, as experience has shown, pressure fluctuations in the melt as it is spun or extruded from the spinneret causes a corresponding undesirable fluctuation in the diameter or denier of the extruded filaments. The resulting filaments with a non-uniform denier would not be satisfactory for most textile applications.

This problem of pressure and denier fluctuations in the spinneret of the spinning extruder is particularly serious when producing finer and finest filaments, fibers or threads, i.e. where the orifice opening in the spinneret has a diameter of less than about 0.25 mm. In this case, the manner in which the spinning pumps convey or dose the polymer through the spinneret is still appreciably influenced by those pressure fluctuations imparted by the extrusion screw, and the spinning pump is unable to provide a constant and uniform amount of the molten polymer to the spinneret. As a result, the fine denier filaments cannot be produced with the high degree of denier uniformity required by the textile industry.

Thus, even though the spinning pump is designed to even out or counteract pressure differences in the molten polymer feed stream, it is still too sluggish to avoid a perceptible transfer of pressure differences from the molten polymer leaving the extrusion screw to the pressure side of the spinning pump and thereby to the spin ning head and orifices of the spinneret. This undesirable result generally occurs regardless of the particular construction of the spinning pump or its exact location in a feed line between the extrusion screw and the spinneret.

In view of this serious problem, various attempts have been made to even out or regulate pressure fluctuations at a point in the flow of the melt preceding the spinning pump, especially where it has been desirable to obtain a uniform denier in spinning very fine denier filaments as noted above. For example, it has been suggested that pressure fluctuations might be dampened in the last working zone of the extrusion screw, usually referred to as the metering zone, by lengthening this zone. However, when resorting to this technique, the expected uniformity of filament or thread denier could not be achieved even though this metering zone was extended to a most unusual length of eight times the diameter of the screw.

Other attempts were then made to provide special pressure regulating devices on the spinning extruder which would presumably function to prevent or counteract major deviations from the desired pressure values in the system. For example, devices have been proposed for automatically modifying or controling the turning speed of the extrusion screw in response to changes of pressure. Similar devices have also been proposed which would automatically control the amount of molten polymer being conveyed by the screw in response to changes of pressure, for example, pressure-regulated valves permitting the dis charge of excess melt or axially slidable crews arranged to control the amount of melt being conveyed at any instant. These regulating devices have not been very satisfactory because they are quite expensive and tend to malfunction. Furthermore, it is most essential to control very small fluctuations in the spinning head, and pressure regulating devices are not always sensitive enough to provide the necessary control.

Finally, it has been suggested that the melt be conveyed from the extrusion screw to a collector or temporary storage vessel and only then to the spinning pumps and spinneret. In this case, however, one is presented with the drawback of considerably increased space requirements and the difiiculties of providing special means for protecting the melt against oxidation. Also, considerably greater heat must be supplied to the intermediate storage vessel in order to maintain the polymer in its molten form, and it is difficult to supply such heat uniformly under these conditions.

In general, all of these proposed solutions to the problem of avoiding pressure fluctuations have a common disadvantage in that they contradict the fundamental requirement of melt spinning, i.e. maintaining as short a residence time as possible for the melt during its passage through the spinning extruder. It is therefore apparent that in spite of the inherent difficulty in solving this problem, an adequate solution is required to improve the quality and uniformity of synthetic polymer filaments, especially in the production of filaments with a relatively fine and uniform denier.

One object of the present invention is to provide novel apparatus, particularly a novel construction of the extrusion screw in the metering zone or portion of said screw, whereby it is possible to avoid pressure fluctuations during the melt spinning of a synthetic thermo plastic polymer into filaments.

Another object of the invention is to provide an improvement in a spinning extruder so as to permit a trouble .3 free melt spinning of fine denier filaments having a more uniform diameter.

Still another object of the invention is to provide an improvement in the extrusion screw of a spinning extruder for the production of uniform denier filaments of a synthetic polymer without substantially increasing the residence time of the molten polymer or exposing the molten polymer to excessive oxidation.

Yet another object of the invention is to provide novel apparatus of the spinning extruder type for the improved melt spinning of a synthetic thermoplastic polymer, wherein the improvement requires only a change in one section of the extrusion screw and can be readily adapted to melt spinning apparatus conventionally employed in this art.

It has now been found, in accordance with the invention, that these and other objects and advantages can be achieved in a surprisingly simple manner if the metering section of the extrusion screw in the melt spinning apparatus, i.e. the spinning extruder, is extended to at least 12 times the diameter of said screw, and the thread depth in this metering section of the screw is enlarged to a value of at least 3.5% of the screw diameter.

The invention is described in more specific detail by the following specification taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a perspective view of a typical spinning extruder illustrating its general appearnce and its principle elements in the overall combination; and

FIG. 2 is a substantially cross-sectional view of a specific embodiment of an extrusion screw illustrating the improvement required by this invention.

In the drawing, the spinning extruder has three essential components including (A) the extrusion screw 1 enclosed by a jacket housing 2, (B) spinning pumps 3 connected by a branched pipeline 4 to the outlet of the extrusion screw, and (C) a spinneret 5 attached directly after each spinning pump. The construction or arrangement of each of these components is generally well known in the art as Well as their specific function in the overall apparatus, so that an elaborate description is not required.

Solid particles of the synthetic thermoplastic fiberforming polymer are introduced into the feed hopper 6 and are first conveyed by the screw 1 through a feed section a in which the thread depth z is usually constant. The polymer is then conveyed through a compression section b in which the polymer mass is preferably completely melted, e.g. by introducing steam or other heatexchange fluid into the jacket 2 surrounding the cylindrical wall 7 of the extrusion screw, and at the same time, the molten polymer is placed under increasing compression by means of a continuously decreasing thread depth t The melt is then further conveyed through the last section c of the extrusion screw, which is usually referred to as the metering zone or may also be called the pump zone or homogenizing zone. The thread depth t in this metering section is usually maintained at a constant value and such construction is also essential for purposes of the present invention.

Ater leaving the extrusion screw, the highly compressed molten polymer is led through any suitable outlet line 4 directly to the spinning pumps 3 where molten polymer is dosed for extrusion through the orifices of the spinnerets 5. By drawing off the extruded filaments at a faster rate than they are extruded, the filaments are stretched for molecule orientation.

The foregoing procedure, although quite conventional, is incapable of producing filaments of uniform denier unless pressure fluctuations can be avoided during spinning or extruding of the molten polymer. In spinning extruders of the prior art and/or in carrying out experiments leading to the present invention, the length of the screw 1 in the metering section was generally considered to be adequate when having a value considerably less than 8 times the screw diameter D and never more than this value. Various thread depths t were suggested for this metering zone, usually in a range of up to not more than about 2.7%, usually about 1.2 to 2.6% of the screw diameter D.

It was therefore quite unexpected to find that an efficient damping or control of pressure peaks could be achieved by simultaneously lengthening the metering section c of the screw l to at least 12 times the screw diameter D and increasing the thread depth z in this section to a value of at least 3.5% up to about 6%, preferably about 4-5%, of the screw diameter D. These dimensions together with certain other dimensions and structural relationships are shown in detail in FIG. 2 of the drawing.

Although the screw length l, in the metering section must be about equal to or more than 12D, it has been found that this length provides satisfactory and even optimum results in controlling pressure fluctuations without any substantial increase in length above 12D, regardless of the remaining dimensions of the overall length L Therefore, the metering length l should ordinarily be maintained within values of approximately 12D to 14D, higher values merely becoming less economical and therefore less desirable.

The overall length L of the screw can be varied between values of about 20D to 27D with optimum results being achieved in the vicinity of 24D. In general it is desirable to construct the screw with a ratio of l aL of at least about 3:7, preferably at least 4:9 and optimally about 1:2. This ratio normally should not exceed 7:10 and is preferably less than 3:5. These values and ratios are directed to the most efficient construction for the purposes of this invention, and the smaller overall lengths L are generally adapted to conditions under which a hot, fluid and relatively less viscous molten material is fed into the feed zone a, whereas the larger overall lengths are employed where a solid coarse-grained and cold polymer is fed into the feed zone a.

Since the metering section I is optimally about 12D or only lightly greater, it will be apparent that the greatest variation in overall length occurs in the feed section 1 and the compression section l Even when feeding a hot, fluid and less viscous molten polymer into the inlet or feed hopper of the extrusion screw, the feed section l should normally be at least equal to the compression section l The feed section l is longer than the compression section l when introducing cold solid particles of the polymer since it is then quite obviously necessary to also heat the particles in the feed section so that they can be at least partially softened and/or melted before passing through the compression zone.

Although an increase in the retention time of the melt in the extrusion screw would normally be expected when substantially increasing the length of the metering zone, this increased retention time is not only prevented by conveying a larger amount of molten polymer for each turn of the screw (as a consequence of increasing the thread depth t but also by increasing the throughput as a result of avoiding pressure fluctuations which otherwise tend to retard the rate of throughput. The specific improvement in construction of the extrusion screw according to the invention is actually capable of decreasing the retention time by as much as 20% by comparison with normal retention time usualy experienced in prior apparatus of this type.

The thread depth z in the metering zone must be at least 0.035D, corresponding to a relatively viscous melt, and can be increased to an upper limit of about 0.06D with a highly fluid or less viscous melt. The thread depths t and t in the compression and feed zones, respectively, are not particularly critical and are generally adapted to the feed material according to well known factors affecting the proper melting and compressing of the polymer to the desired extrusion pressure. Of course, it will be apparent that I must be substantially greater than t and that t must gradually decrease from t, to t in order to increase the pressure on the melt. Similar results can be achieved in the feed and compression zones by using a constant core diameter d while decreasing the inner diameter of the cylindrical wall or screw casing 7 in the compression zone b. In this case, the screw diameter D is constant in zone but increases in zone b and is against constant in zone a. Such modifications provide the same relationships in thread depth and screw length without otherwise departing from the function of the apparatus as required by this invention.

The various dimensional or structural relationship of the extrusion screw in accordance with the invention were calculated with special consideration for the melt spinning of nylon (polycaprolactam or polyhexamethylene adipamide) as the synthetic fiber-forming polymer. Such relationships can be readily adapted to other Well known synthetic thermoplastic fiber-forming polymers in order to obtain optimum results without any substantial departure from the spirit and scope of the invention as defined in this specification and the appended claims.

In a specific working example using the apparatus of the invention, nylon particles were introduced into the extrusion screw I having the following dimensional relationships: L =24D; l =l2Dg and t =4.5% of D. Specific dimensions were as follows:

D=90 mm. 1 :81.9 mm. d =60.0 mm. t =4.05 mm. t =l5.0 mm. l =1080.0 mm. l =540.0 mm. l =540.0 mm.

The number of turns Z of the screw thread per unit length in each of the three zones were as follows:

The molten nylon 6 was maintained at a temperature of about 285 C. in the metering zone c, and the screw was rotated at a speed of 80 revolutions per minute to achieve a nylon throughput of about 130 kg./hr. Extrusion took place through openings of 0.25 mm. in the spinnerets to provide an individual stretch thread denier of 30, consisting of 6 filaments.

In the melt spinning of nylon with the apparatus of the invention, it was not only possible to achieve shorter retention times and considerably less pressure fluctuations but it was apparent that these effects resulted in a much more uniform filament denier. Tests carried out with this improved apparatus clearly show a surprising reduction in the deviation from a standard or constant denier of less than 1%. Although optimum results were achieved with a total screw length L of 24D, a metering zone thread depth of at least 0.035D, preferably 0.04D to 0.05D, and a screw length 1 in the metering section of 12D, similarly successful results were achieved with a total screw length of 20D 01', on the other hand, of 27D.

The invention thus provides a substantial improvement in this particular art by solving the basic problem 01;

greatly diminishing pressure fluctuations so as to maintain the closest possible tolerance of the extruded filament denier while at the same time avoiding any substantial increase in the retention time of the molten polymer dur-' ing its conveyance through the melt spinning apparatus. In fact, this retention time can even be reduced so as to permit higher throughputs as well as higher quality filaments. The apparatus of the invention further avoids any necessity of employing expensive and difficulty maintained pressure regulating devices such as pressure sensitive valves and the like as additional apparatus elements. All of: these advantages are especially valuable in spinning very fine denier filaments having a uniform or constant diameter and adapted to be used in high quality textile applications.

The invention is hereby claimed as follows:

1. In an apparatus for spinning filaments of synthetic thermoplastic fiber-forming polymers including (A) an extrusion screw devided along its total length into a feed section, a compression section and a metering section, (B) a spinning pump adapted to receive molten polymer from said extrusion screw and (C) a spinneret adapted to receive the molten polymer from the spinning pump and containing openings for extrusion of the polymer into filaments, the improvement of a metering section of said extrusion screw which is at least 12 times the diameter of said screw, a thread depth in said metering section which is at least 3.5% of the screw diameter and a total length of the extrusion screw which is equal to about 20 to 27 times the screw diameter.

2. An apparatus as claimed in claim 1 wherein the thread depth in said metering section is at least 3.5% up to about 6% of the screw diameter.

3. An apparatus as claimed in claim 1 wherein the thread depth in said metering section is about 4 to 5% of the screw diameter.

4. An apparatus as claimed in claim 1 wherein the metering section is about 12 to 14 times the screw diameter.

5. An apparatus as claimed in claim 1 wherein said feed section is at least equal in length to said compression section.

6. An apparatus as claimed in claim 1 wherein said metering section is approximately 12 times the screw diameter, the total length of the extrusion screw is approximately 24 times the screw diameter, and the thread depth in the metering section is approximately 4 to 5% of the screw diameter.

References Cited by the Examiner UNITED STATES PATENTS 2,541,201 2/1951 Buecken et al 18-12 X 2,692,405 10/1954 Gayler 18-8 2,705,343 4/1955 Hendry 1830 2,872,703 2/1959 Gambrill et al 1812 3,123,860 3/1964 Vesilind 1812 3,145,420 8/1964 Joukainen et al 1812 3,183,553 5/1965 Slater 18-12 WILLIAM J. STEPHENSON, Primary Examiner. 

1. IN AN APPARATUS FOR SPINNING FILAMENTS OF SYNTHETIC THERMOPLASTIC FIBER-FORMING POLYMERS INCLUDING (A) AN EXTRUSION SCREW DEVIDED ALONG ITS TOTAL LENGTH INTO A FEED SECTION, A COMPRESSION SECTION AND A METERING SECTION, (B) A SPINNING PUMP ADAPTED TO RECEIVE MOLTEN POLYMER FROM SAID EXTRUSION SCREW AND (C) A SPINNERET ADAPTED TO RECEIVE THE MOLTEN POLYMER FROM THE SPINNING PUMP AND CONTAINING OPENINGS FOR EXTRUSION OF THE POLYMER INTO FILAMENTS, THE IMPROVEMENT OF A METERING SECTION OF SAID EXTRUSION SCREW WHICH IS AT LEAST 12 TIMES THE DIAMETER OF SAID SCREW, A THREAD DEPTH IN SAID METERING SECTION WHICH IS AT LEAST 3.5% OF THE SCREW DIAMETER AND A TOTAL LENGTH OF THE EXTRUSION SCREW WHICH IS EQUAL TO ABOUT 20 TO 27 TIMES THE SCREW DIAMETER. 